Signal processor

ABSTRACT

A SIGNAL PROCESSOR WHEREIN BOTH DESIRED AND UNDESIRED SIGNALS ARE PROCESSED SIMULTANEOUSLY TO IMPROVE THE RATIO BETWEEN THE ENERGY OF THE DESIRED SIGNALS TO THE ENERGY OF THE UNDESIRED SIGNALS. ALL SIGNALS BEING PROCESSED DURING A SELECTED PERIOD OF TIME ARE PASSED FIRST THROUGH A FILTER WHEREIN THE FREQUENCY SPECTRUM OF THE UNDESIRED SIGNALS IS MATERIALLY CHANGED AND THE FREQUENCY SPECTUM OF THE DESIRED SIGNALS IS BUT SLIGHTLY CHAGED. THE MATERIAL CHANGE IN THE FREQUENCY SPECTRUM OF THE UNDESIRED SIGNALS, IN ADDITION TO REDUCING THE ENERGY OF THE UNDESIRED   SIGNALS, ENGENDERS AT THE BEGINNING AND END OF THE SELECTED PERIOD OF TIME. SUCH TRANSIENTS ARE DIVERTED TO A DUMMY LOAD ALONG WITH A RELATIVELY SMALL AMOUNT OF THE DEAIRED SIGNALS. THE SWITCH IS ALSO OPERATIVE TO PASS THE REMAINING STEADY STATE PORTION OF BOTH THE DESIRED SIGNALS AND THE REMANENT OF THE UNDERSIRED SIGNALS TO A UTILIZATION DEVICE.

Dec. 12, 1972 Filed Nov. 23, 1970 R. A. CHERWEK SIGNAL PROCESSOR AMPLITUDE AM PLITUDE AMPLITUDE AMPLITUDE AM PLITU DE AMPLITUDE AMPLITUDE 2 Sheets-Sheet Z AMPLITUDE Lml All t M A F/G. 70 a, of I United States Patent 3,706,095 SIGNAL PROCESSOR Roland A. Cherwek, Lynnfield, Mass, assignor to Raytheon Company, Lexington, Mas. Filed Nov. 23, 1970, Ser. No. 91,723 Int. Cl. G015 9/42 US. Cl. 3437.7 4 Claims ABSTRACT OF THE DISCLOSURE A signal processor wherein both desired and undesired signals are processed simultaneously to improve the ratio between the energy of the desired signals to the energy of the undesired signals. All signals being processed during a selected period of time are passed first through a filter wherein the frequency spectrum of the undesired signals is materially changed and the frequency spectrum of the desired signals is but slightly changed. The material change in the frequency spectrum of the undesired signals, in addition to reducing the energy of the undesired signals, engenders transients at the beginning and end of the selected period of time. Such transients are diverted to a dummy load along with a relatively small amount of the desired signals. The switch is also operative to pass the remaining steady state portion of both the desired signals and the remanent of the undesired signals to a utilization device.

BACKGROUND OF THE INVENTION This invention relates generally to signal processors and particularly to signal processors used in Doppler radar systems, such processors being used to distinguish between desired and undesired signals when the frequency spectra of such signals differ.

In continuous wave (C.W.) radar systems moving objects can be detected by measuring a difference in frequency between the frequency of undesirable signals resulting from reflections from stationary objects (meaning objects without any Doppler velocity) and the frequency of desirable signals resulting from reflections from moving objects. This is so because the frequency of the energy reflected by stationary objects is equal to the frequency of the transmitted energy, I while the frequency of the energy reflected by moving objects is foiAf, where A, is the Doppler shift frequency. In other types of systems wherein the received signals are discontinuous functions of time, such signals can be considered to be amplitude modulated signals having a carrier frequency f and f rm, such frequencies being associated respectively with the undesirable signal and desirable signals. The energy contained in such modulated signals, as is well known, is distributed to sideband frequencies in addition to the carrier frequencies. In particular, if the received signals are amplitude modulated by a pulse waveform, the energy contained in such pulse modulated waveform has a set of sideband frequencies distributed in accordance with the well known sin x/x function, each such set centered about its respective carrier frequency. A pulse modulated signal is present in many radar systems, for example, by operation of the range gate in a pulse Doppler radar system or, in a sawtooth frequency modulated continuous wave radar system, by passing the LF. signal through a rejection notch filter 'ice centered at the LF. frequency or a band-pass filter properly centered. In either case it is quite possible, because of the width of the pulse modulating signal and the Doppler shift frequencies of interest, that some of the sideband energy generated by undesirable signals will be at or near those frequencies associated with the desirable signals. When such a condition exists detection of desirable signals (that is, moving objects) is made more difficult than detection of moving objects by C.W. radar systems.

The pulse modulated signals are typically processed by a band-pass filter in combination with amplitude weighting circuitry. The band-pass filter passes substantially all of the energy in signals from moving objects of interest and rejects a substantial portion of the energy in signals from stationary objects. It is noted, however, that although the amount of energy from stationary objects is reduced (thereby minimizing the probability of receiver saturation), the ratio of the energy received from desirable signals to the energy received in sidebands of the undesirable signals is not increased. To improve such ratio, it is known to use amplitude weighting circuitry to shape the pulse modulated signal. Typical amplitude weighting functions are: cosine, cosine squared and Hamming. Unfortunately, known amplitude weighting circuitry is quite complex and sophisticated. As a consequence, the use of such circuitry adds to the initial cost of known systems and makes operation thereof more difficult.

SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a signal processor wherein signals to be processed are simultaneously amplitude modulated in accordance with a discontinuous function of time, some of such signals having a carrier frequency outside a frequency band of interest (so that attenuation of the sideband frequencies associated with such carrier frequency is large) and some of such signals having a carrier frequency within a frequency band of interest (so that attenuation of any frequency associated with such carrier frequency is small).

It is another object of the invention to provide a signal processor for use in a Doppler radar system receiver, the signals to be processed being simultaneously amplitude modulated by a pulse waveform and frequency modulated by Doppler shift frequencies, whereby the detection of the energy reflected by a predetermined range of moving objects is significantly improved over detection by known, more complex, signal processors.

These and other objects of the invention are attained generally by: (1) passing a number of amplitude modulated signals through a notch filter, such filter passing at least one frequency band of interest, the carrier of at least one of such signals being outside the band of interest; and, (2) providing switching means, in circuit with the output of the filter, for dissipating the transient caused by the response of such filter to the amplitude modulated signals, while passing the steady state portion thereof to a utilization device. In particular, in a Doppler radar system receiver the carrier frequency of undesirable signals (that is, stationary objects) is outside the band of interest. Since essentially all the frequency spectrum of desirable signals (that is, those from moving objects) pass through the bandpass filter, without significant change, the level of the steady state signal attributable to such desirable signals is not significantly reduced. On the other hand, since a relatively small amount of the frequency spectrum of undesirable signals (that is, those from stationary objects) pass through the notch filter, the level of the steady state signals attributable to such undesirable signals is significantly reduced. Therefore, because the switching means allows only the steady state portion of the signals from the notch filter to be passed to a utilization device, the ratio of energy of desirable signals to the energy of undesirable signals is larger at the utilization device than the ratio between the energies of such signals at the output of the notch filter.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 shows a pulse Doppler radar employing the principles of the invention;

FIG. 2 shows frequency spectra for signals generated in the radar of FIG. 1',

FIG. 3 shows a time history of a signal generated in the radar of FIG. 1;

FIGS. 4A4D, 5A-5D and 7A-7D show frequency spectra and corresponding time histories of signals generated in the radar of FIG. 1; and

FIG. 6 shows time histories for signals generated in the radar of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, it should be noted first that, for convenience, a pulse Doppler radar has been selected to illustrate how this invention may be applied. However, as will become clear hereinafter, the invention is equally as well applicable to other types of radar, as sawtooth modulated C.W. radar. Thus, the illustrated system includes a clock pulse generator 11, a synchronizer 12, a system trigger generator 13, a transmitter 15, a transmitter oscillator 16, a duplexer 17 and an antenna 19, each of which is conventional in construction and operation to produce, periodically, a directional beam of electromagnetic energy (not shown) to illuminate objects (not shown) within such beam. Echo signals (not shown) from any and all objects are received by antenna 19, and, after passing through the duplexer 17, and RF. amplifier 18, are heterodyned in a signal mixer 21 with a reference signal on line 22. The reference signal is at a frequency f +f such signal here being produced by conventional hereodyning, in a mixer 26, of the signal from oscillator 16 and the signal from COHO oscillator 25. It is noted in passing that an appropriate filtering network (not shown) may be placed in line 22, if desired, to allow only the reference signal to be applied to the signal mixer 21. Since the echo signals are at frequencies f iAf, where A, is the Doppler shift frequency, the signals produced by signal mixer 21 include signals at frequencies fnoiA For expository purposes it will be assumed that echo signals from a single moving object and many stationary objects will be considered. Therefore, the signals produced by signal mixer 21 include undesirable signals of frequency, h (from the stationary objects) and desirable signals of frequency f -l-A, (from the moving object). The frequency spectrum of such signals are qualitatively illustrated in FIG. 2 for the condition that the two are equal.

A range gate 29 of conventional design is provided to enable investigation of echo signals from objects within a certain predetermined range interval. That is, the range gate 29 allows signals from signal mixer 21 to pass to notch filter 33 only when synchronizer 12 transmits a signal, a to enable the range gate 29 for the desired time. The time history of signal a, is shown in FIG. 3. The signals from range gate 29 are, therefore, pulse amplitude modulated signals, the carrier frequency of the signals from the stationary object and the moving objects being, respectively, I and (f l-A The time history and frequency spectra for the range-gated signals are shown in FIGS. 4A to 4D, FIGS. 4A and 4B associated with signals from the stationary objects and FIGS. 4C and 4D associated with signals from the moving object. It is first noted that the frequency spectra for both the signals from stationary objects and the signal from the moving object are of the well known form, sin x/x. The frequency spectrum of the former signals is centered at and the frequency spectrum of the latter signals is centered at f +A However, it should also be noted that portions of sideband frequencies from the signal from the stationary objects occur at or near the frequencies of the signal from the moving object, thereby adversely affecting the detection of the signals from the moving object.

The signals from range gate 29 are transmitted to notch filter 33. This filter, of conventional design, has a frequency response characteristic shown as dotted line 35 in FIGS. 5A and 5C. As shown, the filter significantly attenuates energy at frequencies at and near f but does not significantly attenuate energy at or near f et-A That is, signals within a frequency band of interest (the band of frequencies expected to be generated by the moving object) are not attenuated to any appreciable degree. The corresponding time history of the signals out of notch filter 33 are shown in FIGS. 58 and 5D; FIG. 53 showing the signals from the stationary objects and FIG. 5D showing the signals from the moving object. It is first noted that since essentially all of the frequencies described by the sin x/x function centered at f +a pass through notch filter 33, the amplitude of signals from the moving object is substantially equal to the amplitude of such signals entering the notch filter 33. On the other hand, since notch filter 33 attenuates a significant amount of energy in the signals from the stationary objects, the frequency spectrum of such filtered signals is no longer described by sin x/x. Consequently, the time history of the signals shown in FIG. 58 includes transient portions 36 occurring at the beginning and end of the signal separated by a steady state portion 37. It is noted that the steady state portion 37 of the signals from the stationary objects are at a frequency f and that the amplitude of such steady state portion has been significantly reduced. It is further noted that the sideband frequencies associated with carrier frequency f occurring within the band-pass of the notch filter 33 are, however, not significantly attenuated by such filter.

The signals out of notch filter 33 are transmitted to switch 39 which may be any known electronic switching circuit. The switch 39 is operated, in a manner to be described, to pass the signals out of notch filter 33 either to a dummy load, here represented by resistor 41, or to a utilization device 43. Utilization device 43 may be any conventional frequency spectrum analyzer. The desired switching is controlled by signals a, and a such signals being produced by synchronizer 12. The time history of signals a and a," are shown in FIG. 6. In operation, signal a, is passed to switch 39 simultaneously with signal a;. However, the time duration for the signal a, is much shorter than signal a Signal a," is passed to switch 39 at the end of signal a The length of signals a and a is equal to the transient time associated with the signal shown in FIG. 5B. As is well known, this transient time is a function of the design of the notch filter. During the time period in which signals a, or a are applied to switch 39, the signals processed by notch filter 33 are passed to resistor 41, whereas, in the absence of the signals a, or a the signals roduced by notch filter 33 are passed to utilization device 43. The signals trans mitted to utilization device 43 are shown in FIGS. 73 and 7D, the latter figure showing the time history of the signals produced by the moving object (that is, the

desirable signal) and the former figure showing the time history of the signals produced by the stationary objects (that is, the undesirable signals). Three characteristics must be noted about the signals transmitted to utilization device 43: (l) the amplitude of the signals produced from stationary objects has been reduced more than the amplitude of the signals produced by the moving object; (2) the carrier frequency of the signals is unaltered (this is so because the steady state response of a filter to any pulse modulated signal is a signal at the carrier frequency of such signal); and, (3) the signals are pulse amplitude modulated and, therefore, have frequency spectra described by sin x/ x, the signals from the stationary objects having their frequency spectrum centered at f and the signal from the moving object having its frequency spectrum centered at fna-I-At- The frequency spectra of the signals transmitted to utilization device 43 are shown in FIGS. 7A and 7C, the former figure showing the frequency spectrum of the signals from stationary objects and the latter figure showing the frequency spectrum of signals from the moving object. It is noted that since the energy in the signals from stationary objects has been more significantly reduced than the signals from the moving object, the frequency spectrum centered at i has been significantly reduced. It follows, then, that the sideband frequencies associated therewith have now been significantly attenuated within the frequency band of interest. Therefore, the ratio between the energy of the desired signals to the energy of the undesired signals for the signals applied to utilization device 43 is greater than such ratio between the signals applied to notch filter 33.

While the invention has been described for use in a pulse Doppler radar system, as will now be obvious to one of ordinary skill in the art, the invention can be implemented for use in other Doppler radar systems. For example, the invention can be used in a sawtooth frequency modulated (FM) C.W. radar system. Such a radar system is used, for example, to search for moving airborne objects and to determine the range and velocity of each detected one of such objects by detection of the frequency of beat frequency signals. Such signals may be generated by heterodyning the carrier frequency modulating signals of the transmitted signals with the echo signals in a conventional manner. As is well known, the frequency of the beat frequency signals so obtained provide a measurement of the Doppler frequency shift associated with the reflecting objects and the range between the transmitter and each reflecting object. In using an FM/C.W. radar system to search for airborne objects, it may be assumed that reflections from stationary, that is zero Doppler shift frequency objects (undesirable signals), are produced by objects at relatively short ranges and that reflections from airborne objects (desirable signals) are produced by objects at relatively long ranges. The frequency of the beat frequency signals associated with the former objects are, therefore, significantly lower than the frequency of the beat frequency signals associated with the latter objects. As is well known, however: (1) the modulating signal for the carrier frequency of the transmitted signal has a retrace time associated with each sawtooth function; (2) the terminal portion of the echo signals is heterodyned in the signal mixer with an incorrect reference signal; and, (3) the frequency of the beat frequency signals generated during said terminal portion is significantly larger than the desired portions of the beat frequency signals. The beat frequency signals are passed through a notch filter, such filter being functionally analogous to the notch filter 33 shown in FIG. 1. The function of the notch filter is to: (1) attenuate frequencies associated with zero Doppler shift frequency signals; and (2) attenuate the higher frequencies caused by incorrect heterodyning. Consequently, the signals produced by the notch filter are essentially at the proper beat frequency for the initial portion and at zero amplitude during the terminal portion. In other words, the

signals produced by the notch filter are pulse amplitude modulated signals comprised of signals from stationary objects and signals from moving objects; the carrier frequency of the signals from the stationary objects being significantly attenuated by the notch filter and the carrier frequency associated with the moving objects being passed by the notch filter substantially unaltered. That is, the signals produced by the notch filter are analogous to the signals produced by the notch filter 33 as have been shown in FIGS. SA-SD.

It is felt, therefore, that this invention should not be restricted to the proposed embodiments, but rather should be limited only by the spirit and scope of the following claims.

What is claimed is:

1. A signal processor for increasing the ratio between the energy of desirable signals to the energy of undesirable signals, such desirable and undesirable signals being components of a composite signal to be processed and being distinguishable from each other by differing carrier frequencies, such processor comprising:

(a) means, in circuit with the composite signal to be processed, for pulse modulating such signal whereby each one of the undesirable signals has a carrier frequency outside frequency bands of interest and a portion of its sideband frequencies within the frequency bands of interest, and each one of the desirable signals has substantially all its frequencies inside the frequency bands of interest;

(b) a filter, in circuit with the pulse modulated signal, for significantly attenuating signals at frequencies outside the frequency bands of interest, whereby each one of the undesirable signals, after passing through such filter, has at least one transient portion and a steady state portion and for passing, substantially unattenuated, signals at frequencies within the frequency bands of interest:

(c) a utilization device; and,

(d) switching means, operative during the steady state portion, for coupling signals out of the filter to the utilization device and operative during the transient portion, for decoupling signals out of the filter from the utilization device.

2. In a Doppler radar, a receiver adapted to distinguish between signals reflected from stationary and moving objects, such signals being received substantially simultaneously, the frequency spectrum of signals from the latter being centered at the intermediate frequency of such receiver and the frequency spectrum of signals from the former being centered at a frequency differing from such intermediate frequency, the improvement comprising:

(a) a notch filter having its rejection band centered on the intermediate frequency of the receiver, such filter being operative to attenuate the center frquency of the frequency spectrum of signals from stationary objects and to pass all frequencies in the frequency spectrum of signals from moving objects;

(b) means, operative on the signals out of the notch filter, for selectively attenuating the frequencies in the frequency spectrum of the signals from stationary objects and passing all frequencies in the frequency spectrum of signals from moving objects; and

(c) a utilization device, responsive to the signals out of the just-mentioned means.

3. The improvement recited in claim 2 wherein the means operative on the signals out of the notch filter includes:

(a) a switch which, when actuated, passes signals from the notch filter to the utilization device; and

(b) a synchronizer to actuate the switch when the signals passed by the notch filter are steady state signals.

4. In a Doppler radar, a receiver adapted to distinguish between signals reflected from stationary and moving objects, such signals being received substantially simultaneously, the frequency spectrum of signals from the latter being centered at the intermediate frequency of such receiver and the frequency spectrum of signals from the former being centered at a frequency differing from such intermediate frequency, the improvement comprising:

(a) a filter having its rejection band encompassing the intermediate frequency of the receiver, such filter being operative to attenuate the center frequency of the frequency spectrum of signals from stationary objects and to pass all frequencies in the frequency spectrum of signals from moving objects; (b) means, operative on the signals out of the filter, for

selectively attenuating the frequencies in the frequency spectrum of the signals from stationary ob- References Cited UNITED STATES PATENTS Woerrlein 3437.7 X Fishbein et al 343-7.7 Fishbein et al. 343-7.7 Harrison et a1. 3437.7 Page 3437.7

MALCOLM F. HUBLER, Primary Examiner 

